1. Field of the Invention
The invention relates to a telephone switching system formed on the basis of a switching network designed to switch message information by interconnecting input circuits equipped with programmed-logic units, called input units, while connecting circuits are fitted between these programmed-logic circuits in order to transmit control information.
A system of this kind is described particularly in U.S. Pat. Nos. 4,201,889, 4,201,890 and 4,201,891, all issued May 6, 1980.
2. Description of the Prior Art
In this known system the connecting circuits between the programmed-logic units use the switching network which thus carries simultaneously message information and control information. The establishment of a call between two subscribers connected to the switching network via input circuits involves the exchange of control information between the input units connected to both subscribers: the calling subscriber and the called subscriber. Since the memories forming part of the input units have a limited capacity, ancillary programmed-logic units have to be brought in. To set up a call between a calling subscriber and a called subscriber it is therefore necessary to exchange between different units a stream of control information which utilises the switching network. This switching network is made up of switching elements, also equipped with logic units, which have to effect processing in order to determine the route to be followed within the switching element. Since the establishment of a connection by the switching network involves several switching elements, the processing time and the transmission time will be lengthened as a function of the number of switching elements used. It will thus be appreciated that the establishment of connections between input units at either end takes a certain time, which leads to a first disadvantage of this known system. This first disadvantage is formed by the fact that the establishment of calls takes too much time while busying the switching network. A second disadvantage which is also due to this certain time is that the reloading of the program of the input units takes too long. In fact, the input units have their program loaded in random-access or direct-access memories (RAM's) and, in the event of failure, these memories have to be reloaded via the switching network. Since programs can be fairly bulky and there are numerous memories to be reloaded, the reloading time may become unacceptable.